Batch retreival of ES data with minimal round trips

Batch retreival of ES data with minimal round trips

Batches ES urls by schema and host to reduce round trips required to retreive
many pieces of data.  Takes array of ES urls to retreive and returns a map
from ES urls to their data.  Errored urls are represented with a boolean false
in the result set.  Initially implemented for ExternalStoreDB, other stores
fallback to serial requests.

Change-Id: If1bef25f57bfe7de32fc6787f553a90bd76e87ea